Active Matrix Organic Light Emitting Diode (AMOLED) can emit when being driven by a current generated by a driving Thin-film transistor (TFT) in a saturated state. Because different driving currents will be generated due to different threshold voltages when a same gray scale voltage is put, causing the inconsistency of driving currents. Vth Threshold voltages of transistors Vth of LTPS (Low-temperature Polysilicon) manufacturing process have very bad uniformity, meanwhile. Vth has drifting. The luminance uniformity of such conventional 2T1C circuit has always been bad.
The conventional 2T1C pixel unit driving circuit is shown in FIG. 1. The circuit has only two TFTs, T1 is used as a switch, DTFT is used for driving a pixel. The operation of conventional 2T1C pixel unit driving circuit is also easier. The timing diagram of this 2T1C pixel unit driving circuit is shown in FIG. 2. When a scan level of a scan line Scan is low, T1 is turned on and a gray scale voltage of a data line Data charges a capacitor C. When the scan level is high, T1 is turned off and the capacitor C is used to store the gray scale voltage. Since VDD (voltage outputted from a high level output terminal of a driving power source) is high, DTFT is in a saturated state. A driving current of OLED is I=K(Vsg−|Vth|)2=K(VDD−Vdata−|Vth|)2, wherein Vsg is difference between voltages at a source and a gate of DTFT, Vdata is the gray scale voltage on the data line Data, K is a constant related to transistor size and carrier mobility, once the TFT size and process is determined, K is determined. The equation of the driving current for this 2T1C pixel unit driving circuit contains Vth. As described above, since the LTPS process is not fully developed, even the process parameters are the same, there is great difference between Vth of the TFTs on different positions of the produced panel, causing the different driving currents under the same gray level voltage. Therefore, under this driving scheme, the luminance of different positions of a panel will differ and the luminance uniformity is bad. At the same time, as the usage of OLED panel extends, OLED material ages, causing the threshold voltage of OLED increase. The emitting efficiency of the OLED material decreases and the brightness of the panel decreases with the same current.